Fluorescent lamps are widely used in different environments, such as in homes, offices and industry. Fluorescent lamps are more durable, economical and efficient than incandescent lamps, in which most of the electric power generates heat rather than light. In a conventional fluorescent lamp, the body is a straight tube with a length of about 20 to 60 inches. Fluorescent tubes are low-pressure mercury discharge lamps in which the inner surface of the tube is coated with a fluorescent material. The structure of a fluorescent tube is very simple which has likely contributed to their use for several decades. The lamp consists of an airtight glass tube containing a small amount of mercury, an inert gas, a fluorescent coating such as phosphor, as well as electrodes and a filament at each end of the lamp. At each end of the fluorescent tube, there is a cap with two symmetrically positioned contact pins, to which the electrodes on each side of the tube are connected. DC power to the fluorescent tube is provided via these contact pins.
In order to provide a florescent tube with DC power an AC to DC power supply is used to convert AC line voltage (typically either 115 or 230 volts) to DC input current. The DC current is reduced by the power supply to a level suitable for use in a florescent tube. These power supplies are generally known within the lamp industry as ballasts.
Unlike incandescent or newer light emitting diode (“LED”) lamps, fluorescent lamps will not illuminate or start simply by applying power to the lamp. The lamp requires a starting circuit. The circuit for a fluorescent tube lamp comprises a power supply (ballast) and a starter (capacitor or other switching device). Upon turning on a fluorescent lamp, the resistance through the tube is very high, and the electric current passes through the ballast, the electrodes on one side of the tube and a closed starter circuit. When passing through the electrodes, the current heats the filament, causing it to emit electrons which ionize the gas inside the tube. The ionized gas forms a current path through the tube. When, after a moment, the starter opens, a high voltage spike occurs between the electrodes which causes current conduction through the ionized gas in the fluorescent tube and thus switches on the lamp. Many types of starters are known in the art.
Lighting systems based on LED light sources are a fairly new technology in the lighting field. LED's are desirable because they have substantially longer life and they use far less power than fluorescent tubes of equivalent output. LED replacement tubes for fluorescent lamps are of the same length and diameter of the fluorescent lamp they are intended to replace. LED replacement tubes typically comprise a number of LEDs to produce the desired light. The LEDs are disposed between a heat sink and a clear or translucent cover. The LEDs may be in a series or parallel circuit array. LEDs differ from fluorescent tubes in that only a power supply or ballast capable of converting high voltage AC line current to a relatively lower voltage DC input current to the LEDs is required. No starting circuit is required with LED lamps.
In some types of LED tube lamps, the ballast is built into the lamp. In others, an external ballast is used. As LEDs have become more powerful and continue to gain in wattage, the need has become more critical to dissipate the heat generated by the LEDs. Therefore, in a typical LED tube used for replacing a fluorescent tube approximately ½ of the circumference of the LED tube comprises a metallic heatsink while the other half is clear or translucent for the transmission of light. The LED arrays are thermally, but not electrically, connected to the metallic heat sink.
Government safety regulations require that lighting fixtures be constructed in such a way that when a fluorescent or LED tube is replaced, it is impossible for a user to come into contact with any parts at input voltage even if the lighting fixture were connected to line voltage. With fluorescent tubes, this requirement is met even if a fluorescent tube is replaced in such a way that only one set of contacts is in a tube end connector and a user touches the contacts on the side of the tube during installation. This requirement is met because even though input voltage may be present at the contacts, no current passes through the gas-filled fluorescent tube before the gas in the tube is ionized with a starting pulse. In other words, the gas in the fluorescent tube serves as an insulator in itself. The electric circuit of the fluorescent tube lighting fixture is such that generation of a starting pulse requires that both ends of the tube be connected to the contacts of the tube holder.
The above however, is not true in the case of LED lights. In LED lights current conduction occurs through the tube at any time that one set of contacts is connected to input voltage. To solve this problem, manufacturers of LED tubes have equipped the lights with electronic switches where the switch opens and breaks the electrical circuit when voltage is detected at only one set of contacts. When voltage is detected at both sets of contacts, the switch closes and allows current to flow.
In the present invention, the inventor has recognized that the safety features presently provided by manufacturers of LED tube lamps are inadequate because the metallic heat sink of the tube is not grounded. Under certain failure conditions, the LED arrays could inadvertently make electrical contact with the metallic heat sink causing a short circuit and a potential fire hazard.